1. Field of the Invention
This invention relates generally to a fuel cell system that includes an algorithm for determining whether a plate connection has failed and, more particularly, to a fuel system that uses a cell voltage monitoring sub-system typically used to determine whether a fuel cell in a fuel cell stack is failing to determine whether a connection wire for measuring the voltage potential of a fuel cell has failed.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. The bipolar plates are made of an electrically conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Typically, the voltage output of every fuel cell in the fuel cell stack is monitored to determine its output voltage so that the system knows if a fuel cell voltage is too low, indicating a possible failure. As is understood in the art, because all of the fuel cells are electrically coupled in series, if one fuel cell in the stack fails, then the entire stack will fail. Certain remedial actions can be taken for a failing fuel cell as a temporary solution until the vehicle can be serviced, such as increasing the flow of hydrogen and/or increasing the cathode stoichiometry.
The fuel cell voltages are measured by a cell voltage monitoring sub-system that includes a wire connected to each bipolar plate in the stack and end plates of the stack to measure a voltage potential between the positive and negative sides of each cell. Therefore, a 400 cell stack will include 401 wires connected to the stack.
FIG. 1 is a plan view of a fuel cell system 10 including a fuel cell stack 12 and a cell voltage monitoring sub-system 14. The fuel cell stack 12 includes terminals 16 and 18 at each end of the stack 12 that provide connection locations for the electrical power from the stack 12. The fuel cell stack 12 also includes a series of fuel cells 20 defined by MEAs 22 positioned between bipolar plates 24. The bipolar plates 24 include flow channels for the cathode side of one fuel cell 20 and the anode side of an adjacent fuel cell 20, as discussed above.
The cell voltage monitoring sub-system 14 includes electrical wires 28 where a separate wire 28 is electrically coupled to each bipolar plate 24. The electrical wires 28 for the bipolar plates 24 on opposite sides of an MEA 22 are coupled to the positive and negative input terminals of a differential amplifier 30. The voltage of the cells 20 is measured by subtracting the cells negative plate voltage from the cells positive plate voltage in the amplifier 30, where an output voltage VN indicates the voltage of each cell 20. There are many techniques known in the art for providing this measurement, for example, analog multiplexing.
Because there are so many wires 28 connected to the stack 12, the potential for one of the wires 24 breaking or becoming disconnected from the bipolar plate 24 is a real concern. If one of the wires 28 or plate connections does fail, then the output of the known cell monitoring sub-system provides an indication of a failed cell. However, it would be desirable to distinguish between a low voltage cell and a failed plate connection because the fuel cell system 10 will still operate with a failed plate connection, and it subsequently can be conveniently repaired.